$ D = \left[\begin{array}{rrr}0 & 1 & 2 \\ 1 & 0 & 1 \\ 0 & 1 & 0\end{array}\right]$ What is $ D^{-1}$ ?
$ = \left[\begin{array}{rrr}-\frac{1}{2} & 1 & \frac{1}{2} \\ 0 & 0 & 1 \\ \frac{1}{2} & 0 & -\frac{1}{2}\end{array}\right]$